Frame transfer CCD solid-state imaging devices comprise an imaging section which produces and stores information charges for each pixel as a result of exposure, and a storage section which is light-shielded and holds the information charges transferred at a high speed from the imaging section until the information charges are read out on a single line basis by a horizontal transfer section.
Each of the imaging section and the storage section has vertical CCD registers comprising a plurality of charge-transfer channel regions extended vertically parallel to each other and a plurality of transfer electrodes extended horizontally parallel to each other. Each bit of the CCD shift register includes a plurality of transfer electrodes which are disposed adjacently, and forms at each charge-transfer channel region one potential well for storing information charges as a result of voltage applied to the transfer electrodes. Each bit of the CCD shift register is assigned to a pixel of the imaging device.
Conventional driving circuits form a potential well, which is secured at a fixed location during an exposure period, at each bit of the CCD shift register of the imaging section and causes information charges corresponding to an incident ray volume to be stored in the potential well. That is, ON-voltage is applied to a particular transfer electrode corresponding to a clock of a certain phase among a plurality of transfer electrodes, which are driven by clocks having mutually displaced phases, of each bit, whereby a potential well is formed under the particular transfer electrode.
FIG. 1 schematically shows a potential well during an exposure period according to a conventional driving method when an imaging section comprises a CCD shift register constituted in a three-phase drive. Transfer electrodes 3-1, 3-2 and 3-3 to which clock pulses φi1, φi2 and φi3 are respectively applied are periodically disposed on a charge-transfer channel region 2. One set of the continuously-arranged transfer electrodes 3-1 to 3-3 is assigned to one pixel. FIG. 1 shows lenses 4 which constitute a microlens array. Each lens 4 is disposed on three transfer electrodes 3-1 to 3-3 assigned to a single pixel. During an exposure period, for example, ON-voltage is applied to the transfer electrode 3-2 at the center of the pixel corresponding to the center of the lens, while OFF-voltage is applied to the other transfer electrodes 3-1 and 3-3, as a result of which a potential well 5 is formed under the transfer electrode 3-2 and information charges 6 produced by incident light are stored in the potential well 5.
In the charge-transfer region 2, for example, a dark current occurs due to the effect of an interface state in the vicinity of a surface of a semiconductor substrate. The potential well 5 formed during the exposure period stores not only information charges 6 produced in correspondence with an incident ray but also a dark current generated at a corresponding region. This dark current may deteriorate an S/N ratio. The extent to which dark current is generated depends on uncontrollable factors, such as the interface state, and may fluctuate from place to place in the charge transfer channel region. With conventional driving methods, a dark current component contained in information charges of each pixel is the dark current component generated at the location where the potential well is formed, i.e. the location under the transfer electrode to which the ON-voltage is applied. Since each potential well is formed so as to be spaced two transfer electrodes width from each adjacent potential well, the amount of dark current contained in each potential well is relatively susceptible to the position-dependent variations of the amounts of generated dark current. In other words, the conventional art suffers from a problem where noise on a screen, due to the variations in the amount of dark current component between pixels, tends to become larger, which increases granularity of an image and gives a visual impression that the image appears rough.